Emission standards for unburned hydrocarbons, carbon monoxide and nitrogen oxide contaminants continue to become more stringent. In order to meet such standards, catalytic converters containing a three-way conversion (TWC) catalyst are located in the exhaust gas line of internal combustion engines. Such catalysts promote the oxidation by oxygen in the exhaust gas stream of unburned hydrocarbons and carbon monoxide as well as the reduction of nitrogen oxides to nitrogen.
Many TWC catalysts are manufactured with at least two separate catalyst coating compositions (washcoats) that are applied in the form of aqueous dispersions as successive layers on a substrate (for example, a honeycomb body composed of ceramic or metal) in order to separate noble metals, such as, palladium and rhodium which represent the main catalytically active species. Separation has been necessary historically because palladium and rhodium can form an alloy which is known to be less catalytically active.
TWC catalysts incorporate oxygen storage components (OSC) and alumina materials to support the precious metals. In such TWC catalysts, the activity of Rh can be hindered by interaction with alumina and cerium oxide contained in the OSC composite material. Such interaction can lead to a deactivation of Rh catalytic activity especially when the concentration of the cerium oxide in the OSC composite material exceeds 30 weight %. Furthermore, Rh migrates within the washcoat upon high-temperature aging, i.e. temperature higher than 1000° C. The Rh migration affects negatively the emission performance, in particular the NOx conversion under rich conditions, since Rh would then be in contact with the cerium oxide in the OSC composite material. Rhodium performance can also be hindered by interactions with palladium.
There is a need to provide single washcoat compositions containing both palladium and rhodium while maintaining and/or improving catalytic performance as compared to compositions that provide these metals individually for separate layers. There is also a need for applying the single washcoat composition in one coating step. There is also continuing need to provide a TWC catalyst composites that utilize precious metals efficiently and remain effective to meet regulated HC, NOx, and CO conversions. There is a further need to limit Rh deactivation due to interaction with OSC and to limit the migration of Rh supported materials thus ensuring improved conversion efficiency.